The breathing and swimming activity rates of four bluegills in a laboratory version of a pollution monitoring apparatus increased when the diurnal temperature cycle changed from a control cycle of 24.8–26.0 C to a treatment cycle of 24.8–29.2 C. The latter cycle simulated the temperature regime reported for the Potomac River in June 1961, at a location free of extraneous thermal effluents. If a bluegill response is being used to detect industrial pollution, critical response values must be determined so that a diurnal temperature change does not cause a false warning signal. The results of this investigation suggest that this can be accomplished.